Non-toxic inexpensive synthetic drilling fluid

ABSTRACT

A non-toxic, inexpensive synthetic fluid for use in wellbore fluids (e.g., drilling fluids) is selected from the group consisting of (A) fluids having (I) a pour point greater than about -30° C. (-22° F.) and (II) a cetane index greater than 50, and comprising (i) at least about 95 weight percent hydrocarbons containing 11 or more carbon atoms, (ii) greater than 5 weight percent hydrocarbons containing 18 or more carbon atoms, (iii) at least about 50 weight percent isoparaffins, (iv) at least about 90 weight percent total paraffins, (v) about 1 or less weight percent naphthenics, (vi) less than 0.1 volume percent aromatics, and (vii) at least 2 hydrocarbons containing a consecutive number of carbon atoms, and (B) fluids comprising (i) at least about 95 weight percent hydrocarbons containing 10 or more carbon atoms and (ii) at least about 90 weight percent n-paraffins

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 08/580,112, filedJan. 29, 1996, now U.S. Pat. No. 5,958,845 which is acontinuation-in-part of application Ser. No. 08/494,441, filed Jul. 24,1995, which is a continuation-in-part of application Ser. No.08/422,476, filed Apr. 17, 1995, now U.S. Pat. No. 5,635,457 whichapplications are incorporated herein in their entireties by reference.

BACKGROUND

The present invention relates to wellbore fluids (especially, syntheticfluid-based drilling fluids) and systems and processes for using then ina subterranean formation in oil and gas recovery operations.

While drilling fluids employing synthetic fluids (such aspolyalphaolefin- and ester-based drilling fluids) as the base fluid arecapable of achieving 96 hour LC₅₀ Mysid shrimp (Mysidopois bahia)bioassay test results greater than 100,000 ppm, their commercial use hasbeen severely restricted because of the high cost of the syntheticfluids.

SUMMARY OF THE INVENTION

Accordingly, there is a need for a drilling fluid which employs aninexpensive, non-toxic synthetic fluid as the base fluid. The presentinvention satisfies this need by providing a drilling fluid comprising(a) at least one drilling fluid additive (e.g., an emulsifier, aviscosifier, a weighting agent, and an oil-wetting agent) and (b) aninexpensive, non-toxic base fluid. In one embodiment of the invention,the base fluid is a synthetic fluid having a pour point greater thanabout -30° C. (-22° F.) and comprising (i) at least about 95 weightpercent hydrocarbons containing 11 or more carbon atoms, (ii) greaterthan 5 weight percent hydrocarbons containing 18 or more carbon atoms,(iii) at least about 50 weight percent isoparaffins, (iv) at least about90 weight percent total paraffins, (v) at least 2 hydrocarbonscontaining a consecutive number of carbon atoms, (vi) less than about 1weight percent naphthenics, and (vii) less than about 0.1 volume percentaromatics. (This synthetic fluid is referred to hereinafter as the"isoparaffin synthetic fluid.")

In another embodiment, the synthetic fluid comprises (1) at least about95 weight percent hydrocarbons containing 10 or more carbon atoms and(2) at least about 90 weight percent n-paraffins. (This synthetic fluidis referred to hereinafter as the "n-paraffin synthetic fluid.") Then-paraffins usually also contain (i) less than about 10 weight percentnaphthenics and (ii) less than about 0.1 volume percent aromatics.

Typically, both the isoparaffin and n-paraffin synthetic fluids contain(i) less than about 1 weight percent sulfur, (ii) less than about 1weight percent nitrogen, and (iii) less than about 1 weight percentoxygenated compounds.

The cost of the synthetic fluids employed in the present invention iscomparable to that of diesel because the synthetic fluids are made byreacting inexpensive raw materials (e.g., H₂ and CO) on a massive scaledesigned to supply synthetic substitutes for gasoil and/or keroseneproduced at conventional oil refineries. In contrast, polyalphaolefinsand esters are made by polymerizing or reacting expensive raw materialson a small or moderate scale.

Because prior toxicity studies have shown that aromatics, sulfur,nitrogen, and oxygenated compounds can be toxic, the low orsubstantially non-existent concentrations of these materials in thesynthetic fluids used in the present invention in very desirable. Inaddition, the fluids employed in the present invention which are in factmade synthetically are also desirable in view of anticipatedenvironmental regulations which may restrict the off-shore discharge ofnon-aqueous-base drilling fluids to those drilling fluids using asynthetically produced base fluid.

A drilling system and a method for drilling a borehole are also providedby the invention. The drilling system comprises (a) at least onesubterranean formation, (b) a borehole penetrating a portion of at leastone of the subterranean formations, (c) a drill bit suspended in theborehole, and (d) the above drilling fluid located in the borehole andproximate the drill bit. The drilling method comprises the steps of (a)rotating a drill bit at the bottom of the borehole and (b) introducingthe aforesaid drilling fluid into the borehole (i) to pick up drillcuttings and (ii) to carry at least a portion of the drill cuttings outof the borehole.

DETAILED DESCRIPTION OF THE INVENTION

At least 95 weight percent of the isoparaffin synthetic drilling fluidis commonly composed of compounds containing 11 or more, and morecommonly 12 or more, carbon atom. Also, the isoparaffin synthetic fluidconsists of greater than 5, typically greater than 10, more typicallygreater than 15, even more typically greater than 20, and most typicallygreater than 25, weight percent compounds containing sore than 17 carbonatoms. In fact, compounds containing 18 or more carbon atom canconstitute about 30, 35, 40, 45, or even 50 or more weight percent ofthe isoparaffin synthetic fluid. In addition, the isoparaffin syntheticfluid can contain isoparaffin, naphthenic, aromatic, sulfur, nitrogen,oxygenate, and total paraffin compounds in concentrations independentlyset forth in the following Table I.

                                      TABLE I                                     __________________________________________________________________________    Isoparaffin Synthetic Fluid Composition                                       Iso-                            Total                                                                              Normal                                   paraffin.sup.a,                                                                    Naphthenic.sup.b,                                                                   Aromatic.sup.c,                                                                    Sulfur.sup.d,                                                                      Nitrogen.sup.e,                                                                    Oxygenate.sup.f,                                                                    Paraffin.sup.g,                                                                    Paraffin.sup.h,                          wt % wt %  v %  wt % wt % wt %  wt % wt %                                     __________________________________________________________________________    ≧50                                                                         ≦1                                                                           ≦0.1                                                                        ≦1                                                                          ≦1                                                                          ≦1                                                                           ≧90                                                                         ≧5                                ≧55                                                                         ≦0.5                                                                         ≦0.05                                                                       ≦0.5                                                                        ≦0.5                                                                        ≦0.5                                                                         ≧91                                                                         ≧10                               ≧60                                                                         ≦0.1                                                                         ≦0.01                                                                       ≦0.1                                                                        ≦0.1                                                                        ≦0.1                                                                         ≧92                                                                         ≧15                               ≦65                                                                         ≦0.05                                                                        ≦0.005                                                                      ≦0.05                                                                       ≦0.05                                                                       ≦0.05                                                                        ≧93                                                                         ≧20                               ≦70                                                                         ≦0.01                                                                        ≦0.001                                                                      ≦0.01                                                                       ≦0.01                                                                       ≦0.01                                                                        ≧94                                                                         ≧25                               ≦75                                                                         ≦0.005                                                                       ≦0.0005                                                                     ≦0.005                                                                      ≦0.005                                                                      ≦0.005                                                                       ≧95                                                                         ≧30                               ≦80                                                                         ≦0.001                                                                       ≦0.0001                                                                     ≦0.001                                                                      ≦0.001                                                                      ≦0.001                                                                       ≧96                                                                         ≧35                               ≦85                                                                         ≦0.0005                                                                      --   ≦0.0005                                                                     ≦0.0005                                                                     ≦0.0005                                                                      ≧97                                                                         ≧40                               ≦90                                                                         ≦0.0001                                                                           ≦0.0001                                                                     ≦0.0001                                                                     ≦0.0001                                                                      ≧98                                                                         ≧45                               ≦95                      ≧99                                                                         ≧50                                                               ≧99.5                                  __________________________________________________________________________     .sup.a As determined by UMT 40790.                                            .sup.b As determined by UTM 40790.                                            .sup.c As determined by ASTM D 5186.                                          .sup.d As determined by ASTM D 2622.                                          .sup.e As determined by ASTM D 4629.                                          .sup.f As determined by UTM 484.                                              .sup.g As determined by UMT 40790.                                            .sup.h As determined by UMT 40790.                                       

The pour point of the isoparaffin synthetic fluid (as determined by ASTMD 97) is commonly greater than about -30° C. (-22° F.), more commonlygreater than about -25° C. (-13° F.), even more commonly greater thanabout -20° C. (-4° F.), and most commonly greater than about -15° C. (5°F.). Usually, the pour point of the isoparaffin synthetic fluid is lessthan about 6° C. (43° F.), preferably less than about 3° C. (37° F.),more preferably less than about 0° C. (32° F.), and most preferably lessthan about -3° C. (27° F.).

The flash point of the isoparaffin synthetic fluid (as determined by theCleveland Open Cup method) is at least about 65.6° C. (150° F.),typically at least about 71.1° C. (160° F.), more typically about 76.7°C. (170° F.), even more typically at least about 82.2° C. (180° F.), andmost typically at least about 85° C. (185° F.). Usually, the flash pointof the isoparaffin synthetic fluid is less than about 121.1° C. (250°F.), more typically about 118.3° C. (245° F.) or less, even moretypically about 115.6° C. (240° F.) or less, and most about 112.8° C.(235° F.) or less.

As measured by ASTX D 93, the flash point of the isoparaffin syntheticfluid is at least about 65.6° C. (150° F.), typically at least about71.1° C. (160° F.), more typically about 76.7° C. (170° F.), even moretypically at least about 82.2° C. (180° F.), and most typically at leastabout 85° C. (185° F.), but usually less than about 115° C. (239° F.),more typically about 110° C. (230° F.) or less, even more typicallyabout 105° C. (221° F.) or less, and most about 100° C. (212° F.) orless.

The isoparaffin synthetic fluid frequently has an initial boiling point(as determined by ASTM D 86) of at least about 160° C. (320° F.), morefrequently at least about 165° C. (329° F.), even more frequently atleast about 170° C. (338° F.), and most frequently at least about 175°C. (347° F.) or even at least about 180° C. (356° F.). In addition, theisoparaffin in synthetic fluid commonly has a final boiling point (asdetermined by ASTM D 86) of at least about 340° C. (644° F.), morecommonly at least about 345° C. (653° F.), even more commonly at leastabout 350° C. (662° F.), and most commonly at least about 351° C.(663.8° F.). Furthermore, the final boiling point of the isoparaffinsynthetic fluid is typically about 375° C. (707° F.) or less, moretypically about 370° C. (698° F.) or less, even more typically about365° C. (689° F.) or less, and most typically about 360° C. (680° F.) orless.

The viscosity of the isoparaffin synthetic fluid at 40° C. (104° F.) (asmeasured by ASTM D 445) is ordinarily between about 1 to about 10centistokes (cst). Preferably, the viscosity of the isoparaffinsynthetic fluid at 40° C. (104° F.) is less than about 6, morepreferably less than about 5, even more preferably less than about 4.5,and most preferably less than about 4, cst.

At 15° C., the isoparaffin synthetic fluids commonly have an API gravitygreater than about 40°, more commonly greater than about 42°, even morecommonly greater than about 44°, and most commonly greater than about46°.

The cetane index (as determined by ASTM D 976) is generally greater thanabout 60, preferably greater than about 62, more preferably greater thanabout 64, even lore preferably greater than about 66, and mostpreferably greater than about 68. In fact, the cetane index isfrequently at least about 70, 71, 73, 74, 75, 76, about 77 or more.

An isoparaffin synthetic fluid commercially available from MDS(Malaysia)typically has the properties set forth in the following Table II.

                  TABLE II                                                        ______________________________________                                        Typical Properties of MDS(Malaysia)                                           Isoparaffin Synthetic Fluid                                                              Value                                                              Property     Typical  Max     Min   Test Method                               ______________________________________                                        Density at 15° C., kg/m.sup.3                                                       738      790           ASTM D 1298                               ASTM Color   0        2.0           ASTM D 1500                               Distillation Range, ° C.     ASTM D 86                                 IBP          201              175                                             5%           219                                                              50%          271                                                              90%                   350                                                     95%          353                                                              FBP          358      360                                                     Sulphur, ppm 0        500           ASTM D 1266                               Cetane Index 75               70    ASTM D 976                                Flash Point, ° C.                                                                   88               68    ASTM D 93                                 Pour Point, ° C.                                                                    -7                     ASTM D 97                                 Cloud Point, ° C.                                                                   -2                     ASTM D 2500                               CFPP, ° C.                                                                          -3                     IP 309                                    Kinematic Viscosity                                                                        4.3                    ASTM D 445                                at 40° C., cSt                                                         Aromatics, % v                                                                             <0.1                   ASTM D 5186                               API Gravity at 15° C., °                                                     48.75                                                            ______________________________________                                    

An interesting characteristic of the isoparaffin synthetic fluiddescribed in above Table II is that mono- and poly-methyl isomerstypically constitute at least about 90, more typically at least about92, even more typically at least about 94, and seat typically at leastabout 96, weight percent of the C11 or less isoparaffinic content of theisoparaffin synthetic fluid. In fact, the mono- and poly-methyl isomersof isoparaffins containing 11 or less carbon atoms can constitute 97,98, or even 99, weight percent of the isoparaffin hydrocarbons having upto 11 carbon atoms. In other words, for the isoparaffin synthetic fluidreported in Table II, isoparaffins whose branched moieties contain morethan one carbon atom (e.g., have an ethyl, propyl, butyl, or largersubstituent group) constitute a negligible portion of the total amountof isoparaffins containing 11 or less carbon atoms.

Two other isoparaffin synthetic fluids commercially available fromMDS(Malaysia) typically have the properties set forth in the followingTable III.

                  TABLE III                                                       ______________________________________                                        Typical Properties of Other MDS(Malaysia)                                     Isoparaffin Synthetic Fluids                                                              Value                                                             Property      Fluid A  Fluid B  Test Method                                   ______________________________________                                        Density at 15° C., kg/m.sup.3                                                        738.8    784.2    ASTM D 1298                                   ASTM Color    <0.5     <0.5     ASTM D 1500                                   Distillation Range, ° C. ASTM D 86                                     IBP           207.1    217.4                                                  FBP           351.4    353.8                                                  Sulphur, ppm  >50      >50      ASTM D 1266                                   Cetane Index  77.5     75.5     ASTM D 976-91                                 Flash Point, ° C.                                                                    86       97       ASTM D 93-90                                  Pour Point, ° C.                                                                     <0       <0       ASTM D 97-87                                  Kinematic Viscosity                                                                         3.4      3.5      ASTM D 445                                    at 40° C., cSt                                                         Aromatics, % v                                                                              <0.1     <0.1     UV method                                     ______________________________________                                    

Another isoparaffin synthetic fluid, which is commercially availablefrom Sasol, has the properties shown in the following Table IV.

                  TABLE IV                                                        ______________________________________                                        Typical Sasol Brand                                                           Isoparaffin Synthetic Fluid Properties                                        Property           Value      Test Method                                     ______________________________________                                        Density at 20° C., kg/l                                                                   0.778-0.785                                                                              ASTM D 1298                                     Color, Saybolt     +30        ASTM D 156                                      Distillatian Range at 101.3 kPals                                                                           ASTM D 1078                                     IBP, ° C.   200 min.                                                   FBP, ° C.   250 max.                                                   Sulphur, % mass    <0.01      ASTM D 2622                                     Flash Point        77         IP 170                                          (closed cup at 101.3 kPa), ° C.                                        Kinematic Viscosity at 40° C.                                                             1.6-2.1    ASTM D 445                                      Aromatics, % mass  1 max.     GC                                              Water, % mass      0.01       ASTM D 1744                                     Acidity, mgKOH/G   0.01       ASTM D 3242                                     Ash, % mass        0.01       ASTM D 482                                      Ionol content, mg/kg                                                                             900-1100   GC                                              ______________________________________                                    

When the isoparaffin synthetic fluids are employed as the base fluid ina drilling mud, the base oil generally contains less than 1, preferablyless than about 0.9, more preferably less than 0.8, even more preferablyless than about 0.7, and most preferably less than about 0.6, weightpercent polar activator (e.g., polar ether alcohols). In fact, theconcentration of polar activators in the base fluid is commonly lessthan about 0.5, more commonly less than about 0.4, even more commonlyless than about 0.3, and most commonly less than about 0.2, weightpercent. In addition, the base fluid can contain less than about 0.1,0.05, 0.01, 0.005, 0.001, weight percent polar activator or even betotally devoid of any polar activator. Furthermore, when the bass fluidis the isoparaffin synthetic fluid, the entire drilling mud usuallycontains less than 1, preferably less than about 0.75, more preferablyless than 0.5, even more preferably less than about 0.25, and mostpreferably less than about 0.1, weight percent polar activator. in fact,in such instances the drilling mud can contain less than about 0.05,0.01, 0.005, 0.001, weight percent polar activator or be entirely devoidof any polar activator.

With respect to the n-paraffin synthetic fluid, at least 95 weightpercent of the n-paraffin synthetic drilling fluid is generally composedof compounds containing 10 or more carbon atoms. Typically, at least 95weight percent of the n-paraffin synthetic drilling fluid is composed ofcompounds containing 11 or more, more typically 12 or lore, even moretypically 13 or more, and most typically 14 or more carbon atoms.Usually, the n-paraffin synthetic fluid contains less than about 5, morecommonly less than 3, even more commonly less than about 2, and mostcommonly less than about 1, weight percent of compounds containing 18 ormore carbon atoms. In addition, the n-paraffin synthetic fluid cancontain n-paraffin, iso-paraffin, naphthenic, aromatic, sulfur,nitrogen, and oxygenate compounds in concentrations independently listedin the following Table V.

                                      TABLE V                                     __________________________________________________________________________    N-paraffin Synthetic Fluid Composition                                        N-paraffin.sup.a,                                                                   Naphthenic.sup.b,                                                                   Aromatic.sup.c,                                                                    Sulfur.sup.d,                                                                      Nitrogen.sup.e,                                                                    Oxygenates.sup.f,                                                                   Isoparaffins.sup.g,                          wt %  wt %  v %  wt % wt % wt %  wt %                                         __________________________________________________________________________    ≧90                                                                          ≦10                                                                          ≦0.1                                                                        ≦1                                                                          ≦1                                                                          ≦1                                                                           ≦10                                   ≧91                                                                          ≦5                                                                           ≦0.05                                                                       ≦0.5                                                                        ≦0.5                                                                        ≦0.5                                                                         ≦9                                    ≧92                                                                          ≦1                                                                           ≦0.01                                                                       ≦0.1                                                                        ≦0.1                                                                        ≦0.1                                                                         ≦8                                    ≧93                                                                          ≦0.5                                                                         ≦0.005                                                                      ≦0.05                                                                       ≦0.05                                                                       ≦0.05                                                                        ≦7                                    ≧94                                                                          ≦0.1                                                                         ≦0.001                                                                      ≦0.01                                                                       ≦0.01                                                                       ≦0.01                                                                        ≦6                                    ≧95                                                                          ≦0.05                                                                        ≦0.0005                                                                     ≦0.005                                                                      ≦0.005                                                                      ≦0.005                                                                       ≦5                                    ≧96                                                                          ≦0.01                                                                        ≦0.0001                                                                     ≦0.001                                                                      ≦0.001                                                                      ≦0.001                                                                       ≧4                                    ≧97                                                                          ≦0.005                                                                            ≦0.0005                                                                     ≦0.0005                                                                     ≦0.0005                                                                      ≧3                                          ≦0.001                                                                            ≦0.0001                                                                     ≦0.0001                                                                     ≦0.0001                                                                      ≧2                                          ≦0.0005             ≧1                                          ≦0.0001                                                          __________________________________________________________________________     .sup.a As determined by UTM 40790.                                            .sup.b As determined by UTM 40790.                                            .sup.c As determined by ASTM D 5186.                                          .sup.d As determined by ASTM D 2622.                                          .sup.e As determined by ASTM D 4629.                                          .sup.f As determined by UTM 484.                                              .sup.g As determined by UTM 40790.                                       

The pour point of the n-paraffin synthetic fluid (as determined by ASTMD 97) is commonly greater than about -30° C. (-22° F.) and more commonlygreater than about -25° C. (-13° F.). Frequently, the pour point of then-paraffin synthetic fluid is less than about 10° C. (50° F.), morefrequently less than about 9° C. (48.2° F.), even more frequently lessthan about 8° C.(46.4° F.), and most frequently less than about 7° C.(44.6° F.).

The flash point of the n-paraffin synthetic fluid (as determined by ASTMD 93) is typically at least about 65° C. (149° F.), more typically atleast about 70° C. (158° F.), even more typically at least about 75° C.(167° F.), and most typically at least about 80SC (176° F.). Then-paraffin synthetic fluids can have even higher flash points, such asat least about 85° C. (185° F.), 90° C. (194° F.), 95° C. (203° F.), orat least about 100° C. (212° F.) or higher.

The n-paraffin synthetic fluid frequently has an initial boiling point(as determined by ASTM D 86) of at least about l90° C. (374° F.), morefrequently at least about 200° C. (392° F.), even more frequently atleast about 210° C. (410° F.), and most frequently at least about 220°C. (428° F.). Even higher initial Soiling points, such as about 230° C.(446° F.), 240° (464° F.), or 250° C. (482° F.) or more, are not unusualfor the n-paraffin synthetic fluids.

The viscosity of the n-paraffin synthetic fluid at 40° C. (104° F.) (asmeasured by ASTM D 445) is ordinarily between about 1 to about 10 cst.Preferably, the viscosity of the n-paraffin synthetic fluid at 40° C.(104° F.) is less than about 5, more preferably less than about 4, evenmore preferably less than about 3, and most preferably legs than about2, cut.

At 15° C., the n-paraffin synthetic fluids commonly have an API gravitygreater than about 45°, more commonly greater than about 50°, even morecommonly greater than about 50.5°, and most commonly greater than about51°.

Typical properties for some commercially available n-paraffin syntheticfluids are shown in the following Tables VI and VII.

                  TABLE VI                                                        ______________________________________                                        Typical Properties of MDS(Malaysia) Brand                                     N-paraffin Synthetic Fluids                                                   Property      Sarapar 103                                                                             Sarapar 147                                                                             Test Method                                 ______________________________________                                        Saybolt Color +30       +30       ASTM D 156                                  Bromine Index,                                                                              8         6         ASTM D 2710                                 mgBr/100 g                                                                    Sulphur, ppm  0         0         ASTM D 5120                                 Carbon Distribution,                                                          % mass                                                                        nC9           0         0                                                     nC10          9         0                                                     nC11          30        0                                                     nC12          29        0                                                     nC13          27        4                                                     nC14          1         25                                                    nC15          0         24                                                    nC16          0         22                                                    nC17          0         16                                                    nC18          0         4                                                     nC19          0         0                                                     n-Paraffin content, % mass                                                                  96        95                                                    Average Molecular Mass                                                                      167       213                                                   Density at 15° C., kg/m.sup.3                                                        750       775       ASTM D 4052                                 Distillation Range, ° C.   ASTM D 86                                   IBP           190       250                                                   FBP           230       280                                                   Flash Point, ° C.                                                                    75        110       ASTM D 93                                   Pour Point, ° C.                                                                     -20       5         ASTM D 97                                   Viscosity at 25° C., mm/s                                                            1.7       3.3       ASTM D 445                                  API Gravity at 15° C., °                                                      57.17     51.08                                                 ______________________________________                                    

                                      TABLE VII                                   __________________________________________________________________________    Typical Properties Of Sasol Brand N-paraffin Synthetic Fluids                              Paraffins                                                                     Light      Heavy                                                 Property     Specification                                                                       Typical                                                                            Specification                                                                       Typical                                                                            Test Method                                __________________________________________________________________________    n-Paraffins, % mass                                                                        92 min.                                                                             93   92 min.                                                                             93   Sasol 11.28/83                             Aromatics, % mass                                                                          0.5 max.                                                                            <0.1 0.5 max.                                                                            <0.1 Sasol 5.107/92                             Bromine Index, mgBr/100 g                                                                  20 max.                                                                             <10  20 max.                                                                             <10  ASTM D 2710-89                             Sulphur, ppm 5 max.                                                                              <1   5 max.                                                                              <1   ANTEK 1.211/92                             Acidity, mg KOH/g                                                                          0.02 max.                                                                           <0.01                                                                              0.02 max.                                                                           <0.01                                                                              ASTM D 3242                                Ash, % mass  0.03 max.                                                                           <0.01                                                                              0.03 max.                                                                           <0.01                                                                              ASTM D 482                                 Saybolt Color                                                                              +30 min.                                                                            +30  +25 min                                                                             +30  ASTM D 156                                 Carbon Distribution, % mass        Sasol 11.28/83                             C 9 and lighter                                                                            0.5 max.                                                                            <0.1                                                       C10          4-10  5                                                          C11          30-38 35                                                         C12          29-37 32                                                         C13          23-30 28                                                         C14 and heavier                                                                            0.5 max.                                                                            0.2                                                        C13 and lighter         0.5 max.                                                                            3.5                                             C14 and heavier                                                                            0.5 max.                                                                            0.2                                                        C13 and lighter         0.5 max.                                                                            3.5                                             C14-C17                 95 min.                                                                             96                                              C18 and heavier         1 max.                                                                              0.3                                             Boiling Range, ° C.                                                                       192-226    254-287                                         Pour Point, ° C.                                                                          <0         3                                               Flash Point, ° C.                                                                         70         114                                             Average Molecular Mass                                                                           163        219                                             Density at 25° C., kg/l                                                                   0.744                                                      API Gravity at 25° C., °                                                           58.43                                                      Viscosity at 40° C., cst                                                                             2.4                                             __________________________________________________________________________

The synthetic fluids of the present invention are prepared by theFischer-Tropsch process and various modifications thereof (especiallythe Shell Middle Distillate Synthesis process). See, for example, Sie etal., Catalysis Today, 1:371-394 (1991); van der Burgt et al., PetroleumReview, pages 204-209 (April 1990); Oil & Gas Journal, pages 74-76 (Feb.17, 1986); Eilers et al., Catalysis Letters, pages 253-270 (1990);Bartholomew, Catalysis Letters, pages 303-316 (1990); Gregor, CatalysisLetters, pages 317-332 (1990); Dry, Journal of Organometallic Chemistry,372:117-127 (1989); Dry, Applied Industrial Catalysis, 2:167-213 (1983);and Dry, Hydrocarbon Processing, pages 121-124 (August 1982), thesepublications being incorporated herein in their entireties by reference.In general, the Fischer-Tropsch process entails reacting carbon monoxideand hydrogen over a catalyst (e.g., iron, ruthenium, or cobalt) toproduce products which, in the absence of secondary transformations, arehighly linear. When desired, some or all of the linear products aresubjected to a conversion process (such as the Shell Middle DistillateSynthesis Process) where (a) olefins present in the Fischer-Tropschproduct are hydrogenated, (b) small amounts of oxygen-containingcompounds, mainly primary alcohols, are removed, (c) the'Fischer-Tropsch product is hydroisomerized, and (d) the n-paraffins arehydrocracked to isoparaffins of a desired chain length and/or boilingrange.

Due to the manner in which they are synthesized, the synthetic fluidsare composed of hydrocarbons containing a consecutive number of carbonatoms (i.e., a mixture of hydrocarbons where the carbon atom content ofthe individual hydrocarbons is C_(n), C_(n+1), C_(n+2), C_(n+3), etc.and n is a whole number.) Generally, the synthetic fluids are composedof at least 2, more commonly at least 3, even more commonly at least 4,and most commonly at least 5 hydrocarbons containing a consecutivenumber of carbon atoms. In fact, some synthetic fluids contain at least6, 7, 8, 9, or 10 or more hydrocarbons having a consecutive number ofcarbon atoms.

The synthetic fluids are commercially available from Sasol in SouthAfrica and Shell Middle Distillate in Malaysia and are preferably thefraction which has a boiling range similar to gasoils and/or kerosenesproduced at conventional petroleum refineries. optionally, one or morepour point depressants are employed in the synthetic fluids of thepresent invention to lower their pour point. Typical pour pointdepressants include, but are not limited to, ethylene copolymers,isobutylane polymers, polyaklylnaphthalenes, wax-aromatic condensationproducts (e.g., wax-naphthalene condensation products, phenol-waxcondensation products), polyalkylphenolesters, polyalkylmethacrylates,polymethacrylates, polyalkylated condensed aromatics, alkylaromaticpolymers, iminodiiuides, and polyalkylstyrene. (The molecular weightsfor polyaklylnaphthalenes, polyalkylphenolesters, andpolyalkylmethacrylates range from about 2,000 to about 10,000.) Becausethey are non-toxic, ethylene copolymers and isobutylene polymers are thepreferred pour point depressants.

Up to about 1 weight percent pour point depressant is employed. (As usedin the specification and claims, the weight percent of the pour pointdepressant is based upon the weight of the synthetic fluid, i.e., it isthe weight of the pour point depressant divided by the weight of thesynthetic fluid, the quotient being multiplied by 100%.) Preferably, thepour point depressant is employed in a concentration of 0.005 to about0.5, more preferably about 0.01 to about 0.4, and most preferably about0.02 to about 0.3, weight percent.

When employed, the pour point depressant is preferably mixed with thesynthetic fluid and the resulting composition is then combined with anyadditional additives as described below.

One or more surfactants (e.g., emulsifiers, wetting agents),viscosifiers, weighting agents, fluid loss control agents, and shaleinhibiting salts are also optionally used in the drilling fluid of thepresent invention. (As used in the specification and claims, the term"surfactant" means a substance that, when present at low concentrationin a system, has the property of adsorbing onto the surfaces orinterfaces of the system and of altering to a marked degree the surfaceor interfacial free energies of those surfaces (or interfaces). As usedin the foregoing definition of surfactant, the term "interface"indicates a boundary between any two immiscible phases and the term"surface" denotes an interface where one phase is a gas, usually air.)Because the drilling fluids of the present invention are intended to benon-toxic, these optional ingredients, like the synthetic fluid, arepreferably also non-toxic.

Exemplary emulsifiers include, but are not limited to, fatty acids,soaps of fatty acids, and fatty acid derivatives including amido-amines,polyamides, polyamines, esters (such as sorbitan monoleatepolyethoxylate, sorbitan dioleate polyethoxylate), isidaxolines, andalcohols.

Typical wetting agents include, but are not limited to, lecithin, fattyacids, crude tall oil, oxidized crude tall oil, organic phosphateesters, modified isidazolines, modified asidoamines, alkyl aromaticsulfates, alkyl aromatic sulfonates, and organic esters of polyhydricalcohols.

Exemplary weighting agents include, but are not limited to barite, ironoxide, gelana, siderite, and calciun carbonate.

Common shale inhibiting salts are alkali metal and alkaline-earth metalsalts. Calcium chloride and sodium chloride are the preferred shaleinhibiting salts.

Exemplary viscosifiers include, but are not limited to, organophilicclays (e.g., hectorite, bentonite, and attapulgite), non-organophilicclays (e.g., montmorillonite (bentonite), hectorite, saponite,attapulgite, and illite), oil soluble polymers, polyamide resins, andpolycarboxylic acids and soaps. (As used in the specification andclaims, the term "non-organophilic clay" means a clay which has not beenamine-treated to convert the clay from water-yielding to oil-yielding.)

Illustrative fluid loss control agents include, but are not limited to,asphaltics (e.g., asphaltenes and sulfonated asphaltenes), amine treatedlignite, and gilsonite. For drilling fluids intended for use in hightemperature environments (e.g., where the bottom hole temperatureexceeds about 204.4° C. (400° F.)), the fluid loss control agent ispreferably a polymeric fluid loss control agent. Exemplary polymericfluid loss control agents include, but are not limited to, polystyrene,polybutadiene, polyethylene, polypropylene, polybutylene, polyisoprene,natural rubber, butyl rubber, polymers consisting of at least twomonomers selected from the group consisting of styrene, butadiene,isoprone, and vinyl carboxylic acid. Individual or mixtures of polymericfluid loss control agents can be used in the drilling fluid of thisinvention.

For drilling fluids intended for use in high temperature environments(e.g., where the bottom hole temperature exceeds about 204.4° C. (400°F.)), it is desirable to use the synthetic fluid as the base material inconjunction with the formulations and materials disclosed in U.S. patentapplications Ser. No. 07/786,034 and Ser. No. 08/268,801, whichapplications are incorporated herein in their entireties by reference.

General drilling fluid formulations are set forth in the following TableVIII:

                  TABLE VIII                                                      ______________________________________                                                                     More                                             Ingredient       Typical     Typical                                          ______________________________________                                        Synthetic fluid, volume %.sup.a                                                                25-85       40-60                                            Surfactant (active), ppb.sup.b                                                                 0.5-40      3-25                                             kg/m.sup.3       2.86-57.2   2.86-28.6                                        Water, volume %.sup.a                                                                          up to 45    1-20                                             Weighting agent, pp.sup.b                                                                      up to 700   150-600                                          kg/m.sup.3       up to 2002  429-1716                                         Polymer viscosifier, ppb                                                                       0.05-15     0.1-6                                            kg/m.sup.3       0.143-42.9  0.286-17.16                                      Organophilic clay, ppb                                                                         up to 15    0.1-6                                            kg/m.sup.3       up to 42.9  0.286-17.16                                      Shale inhibiting salt, ppb                                                                     up to 60    5-30                                             kg/m.sup.3       up to 171.6 143-85.8                                         Lime.sup.c, ppb  up to 30    1-20                                             kg/m.sup.3       up to 85.8  2.86-57.2                                        Fluid loss control agent, ppb                                                                  up to 30    2-20                                             kg/m.sup.3       up to 85.8  5.72-57.2                                        ______________________________________                                         .sup.a Volume percent is based on the total volume of the drilling fluid.     .sup.b The pounds per barrel (ppb) is based upon the final composition of     the drilling fluid.                                                           .sup.c As used in the specification and claims, the term "lime" means         quicklime (CaO), quicklime precursors, and hydrated quicklime (e.g.,          slaked lime (Ca(OH).sub.2)).                                             

The properties (e.g., synthetic fluid to water ratio, density, etc.) ofthe drilling fluids of the invention can be adjusted to suit anydrilling operation. For example, the drilling fluid is usuallyformulated to have a volumetric ratio of synthetic fluid to water ofabout 100:0 to about 40:60 and a density of about 0.9 kg/l (7.5 poundsper gallon (ppg)) to about 2.4 kg/l (20 ppg). More commonly, the densityof the drilling fluid is about 1.1 kg/l (9 ppg) to about 2.3 kg/l (19ppg). 5 The drilling fluids are preferably prepared by mixing theconstituent ingredients in the following order: (a) synthetic fluid, (b)emulsifier, (c) line, (d) fluid lose control agent, (e) an aqueoussolution comprising water and the shale inhibiting salt, (f)organophilic clay (when employed), (g) oil wetting agent, (h) weightingagent, (i) non-sulfonated polymer, (j) sulfonated polymer (whenemployed), and (k) non-organophilic clay (when employed).

EXAMPLES

The following examples (which are intended to illustrate and not limitthe invention defined by the claims) demonstrate the preparation ofexemplary drilling fluids within the scope of the present invention(Examples 1-7), show the results obtained from an analysis of anisoparaffin synthetic fluid sample (Example 8), document initial andaged Theological properties of a drilling fluid which employs theisoparaffin synthetic fluid sample as the base fluid (Example 9), andcompare the toxicity of two drilling fluids which solely differ in thatthe base fluid of one is the isoparaffin synthetic fluid sample and thebase fluid of the other is the dimer of 1-decene (a commercially used,non-toxic base fluid).

Example 1-6 Preparation of Drilling Fluids

Six drilling fluids (3 lab barrels per drilling fluid formulation, witheach lab barrel containing about 350 ml) having a density of about 2.16kg/l (about 18 ppg) and within the scope of the present invention areformulated by sequentially adding ingredients in the order set forth inTable A. After the addition of each ingredient, the resultingcomposition is mixed for the indicated mixing time prior to adding asubsequent ingredient to the composition.

                                      TABLE A                                     __________________________________________________________________________                 Examples          Mixing Time,                                   Component    1  2  3  4  5  6  minutes                                        __________________________________________________________________________    Synthetic fluid, ml                                                                        164.5                                                                            164.5                                                                            164.5                                                                            164.5                                                                            164.5                                                                            164.5                                             Primary emulsifier, ml                                                                     8.5                                                                              8.5                                                                              8.5                                                                              8.5                                                                              8.5                                                                              8.5                                               Lime, g      8.0                                                                              8.0                                                                              8.0                                                                              8.0                                                                              8.0                                                                              8.0                                               Fluid loss control agent, g                                                                10.0                                                                             10.0                                                                             10.0                                                                             10.0                                                                             10.0                                                                             10.0                                                                             20                                             Brine solution                 10                                             CaCl.sub.2, g                                                                              12.1                                                                             12.1                                                                             12.1                                                                             12.1                                                                             12.1                                                                             12.1                                              Water, ml    23.8                                                                             23.8                                                                             23.8                                                                             23.8                                                                             23.8                                                                             23.8                                              Organophilic clay, g                                                                       1.0                                                                              2.0                                                                              3.0                                                                              2.0                                                                              2.0                                                                              1.0                                                                              20                                             Oil wetting agent, ml                                                                      6.0                                                                              6.0                                                                              6.0                                                                              6.0                                                                              6.0                                                                              6.0                                                                              10                                             Weighting Agent, g                                                                         572                                                                              572                                                                              572                                                                              572                                                                              572                                                                              572                                                                              20                                             Styrene-butadiene polymer, g                                                               0  0  0  0  0  2.0                                                                              10                                             Polystyrene, g                                                                             3.0                                                                              3.0                                                                              3.0                                                                              4.0                                                                              2.0                                                                              3.0                                                                              10                                             Bentonite, g 3.0                                                                              3.0                                                                              3.0                                                                              2.0                                                                              4.0                                                                              3.0                                                                              35                                             __________________________________________________________________________

Example 7 Preparation of Drilling fluid

An invert emulsion drilling fluid is prepared by (a) initially agitatingabout 240 ml of a synthetic fluid for about 1 minute using a blender and(b) then sequentially adding the following ingredients (with continuousmixing for about one minute after the addition of each material): (i)about 6 g of a primary emulsifier; (ii) about 8 g of lime (calciumhydroxide); and (iii) about 4 g of a fluid-loss preventing agent.

Subsequently, about 39 ml of fresh water is added to the above mixtureand the resulting composition is mixed for about ten minutes. Then,about 11 g of an amine-treated bentonite is added and the resultingmixture is agitated for about 15 minutes.

Thereafter, the following materials are added in sequence, with about 5minutes of mixing after the addition of each of the materials: (i) about2 g of a secondary emulsifier; (ii) about 210 g of powdered barite (anon-toxic weighting agent); (iii) about 24 g of calcium chloridedihydrate (to provide salinity to the water phase without water wettingthe barite); and (iv) about 20 g of a powdered clay (composed of about35 weight percent smectic and about 65 weight percent kaolinite) tosimulate drilled formation particles.

Example 8 Analysis of Isoparaffin Synthetic Fluid Sample

Analytical results obtained from the analysis of an isoparaffinsynthetic fluid sample from Shell Malaysia by gas chromatography arereported in the following Tables B-D.

                  TABLE B                                                         ______________________________________                                        Carbon Number Distribution Results                                            Carbon Number  Weight Percent Off                                             ______________________________________                                        8              0.31                                                           9              0.41                                                           10             0.84                                                           11             1.86                                                           12             4.61                                                           13             6.22                                                           14             7.13                                                           15             8.33                                                           16             8.38                                                           17             9.43                                                           18             11.04                                                          19             10.95                                                          20             10.39                                                          21             8.23                                                           22             5.95                                                           23             3.43                                                           24             1.51                                                           25             0.64                                                           >25            0.34                                                           ______________________________________                                    

                  TABLE C                                                         ______________________________________                                        Paraffin Distribution Results                                                 Analytical Data Calculated Results                                            Carbon  n-Paraffin  Iso-Paraffin                                                                            Ratio of                                        Content Wt. %       Wt. %     Iso/n Paraffin                                  ______________________________________                                        10      0.60        0.24      0.40                                            11      1.55        0.31      0.20                                            12      2.60        2.01      0.773                                           13      2.83        3.39      1.198                                           14      5.22        1.91      0.366                                           15      4.70        3.63      0.77                                            16      4.30        4.01      0.949                                           17      4.69        4.74      1.01                                            18      4.52        6.52      1.44                                            19      3.33        7.62      2.29                                            20      2.25        8.14      3.62                                            21      1.53        6.70      5.17                                            22      0.89        5.06      5.68                                            23      0.39        3.04      7.79                                            24      0.12        1.39      11.58                                           25      0.03        0.61      20.33                                           26      0.01        0.33      33                                              27      0.00                                                                  28      0.00                                                                  Total   39.53                                                                 ______________________________________                                    

Based upon the results listed in Table C, the iso-paraffin/n-paraffinratio of the isoparaffin synthetic fluid sample for compounds containingfrom 17 to 20 carbon atoms follows the equationy=(x-16).sup.(0.53+0.2(x-18)), where x is the carbon number and y is theiso-paraffin/n-paraffin ratio. In addition, for compounds containing 21to 25 carbon atoms, the iso-paraffin/n-paraffin ratio of the isoparaffinsynthetic fluid sample follows the equation y=(x-21) .sup.(1.48+0.25(x-23)), where x and y are as defined above. The foregoing equations aregenerally accurate within ±1 unit and even within ±0.5 unit.

                  TABLE D                                                         ______________________________________                                        Percent Off To Temperature Results                                                   Temperature           Temperature                                      Wt % Off ° C.                                                                            ° F.                                                                          V % Off   ° C.                                                                        ° F.                           ______________________________________                                        0.5      150.0    302    0.5       145.6                                                                              294                                   5.0      207.8    406    5.0       205.6                                                                              402                                   10.0     222.8    433    10.0      221.1                                                                              430                                   15.0     237.8    460    15.0      236.7                                                                              458                                   20.0     253.9    489    20.0      252.8                                                                              487                                   25.0     260.6    501    25.0      257.8                                                                              496                                   30.0     272.2    522    30.0      271.7                                                                              521                                   35.0     281.7    539    35.0      280.0                                                                              536                                   40.0     289.4    553    40.0      288.9                                                                              552                                   45.0     298.9    570    45.0      297.2                                                                              567                                   50.0     304.4    580    50.0      304.4                                                                              580                                   55.0     311.7    593    55.0      310.6                                                                              591                                   60.0     318.3    605    60.0      317.8                                                                              604                                   65.0     323.9    615    65.0      322.8                                                                              613                                   70.0     331.7    629    70.0      330.6                                                                              627                                   75.0     337.2    639    75.0      335.6                                                                              636                                   80.0     345.0    653    80.0      344.4                                                                              652                                   85.0     351.7    665    85.0      351.1                                                                              664                                   90.0     360.0    680    90.0      359.4                                                                              679                                   95.0     371.7    701    95.0      371.1                                                                              700                                   99.5     399.4    751    99.5      398.9                                                                              750                                   ______________________________________                                    

In addition, the gas chromatography analysis (both mass spectrometry andflame ionization detector (FID)) did not detect the presence of eitheraromatic or naphthenic compounds

Example 9 Preparation And Testing of Isoparaffin SyntheticFluid-Containing Drilling Fluid

Each of two substantially identical samples of an oil-base drillingfluid within the scope of the present invention was formulated asfollows. (The isoparaffin synthetic fluid sample analyzed in Example 8was employed as the synthetic fluid.) Ingredients were sequentiallyadded in the order set forth below in Table E. After the addition ofeach ingredient, the resulting composition was mixed for the indicatedmixing time prior to adding a subsequent ingredient to the composition.

                  TABLE E                                                         ______________________________________                                                                Mixing Time,                                          Component               minutes                                               ______________________________________                                        Synthetic Fluid, ml  209                                                      Imvitone brand organophilic clay, g                                                                7.0    30                                                Novamul brand emulsifier, ml                                                                       10.0                                                     Novamod brand rheology modifier, g                                                                 2.0    10                                                Lime, g              10.0   10                                                Brine solution              30                                                CaCl.sub.2, g        26.3                                                     Water, ml            51. 3                                                    Versatrol I brand fluid loss                                                  control agent, g     10.0   15                                                Barite, g            269    30                                                ______________________________________                                    

One sample was used to check the initial rheological properties, and theother sample was used to test the aged Theological properties. Theage-tested sample was placed into an aging bomb in the presence of about790.8 kpascal (100 psi) nitrogen and rolled at about 176.7° C. (350°F.). after aging, the rheological properties of the age-tested samplewere checked. Unless otherwise noted below in Table F, both the initialand age-tested rheological properties were measured at about at 48.9° C.(120° F.) according to procedures described in RecommendedPractice--Standard Procedure for Field Testing Drilling Fluids, APIRecommended Practice 13B-2 (RP 13B-2), Second Edition, Dec. 1, 1991,American Petroleum Institute, Washington, DC (hereinafter referred to as"API"), API being incorporated herein in its entirety by reference. Themeasured results are set forth in Table F.

                  TABLE F                                                         ______________________________________                                        Drilling Fluid Rheological Properties                                         Property          Initial                                                                              Aged                                                 ______________________________________                                        Dial reading.sup.a,                                                           600 rpm           103    106                                                  300 rpm           74     61                                                   200 rpm           59     48                                                   100 rpm           42     29                                                   6 rpm             20     8                                                    3 rpm             18     7                                                    Gel Strength.sup.b,                                                           10 sec, lb/100 ft.sup.2                                                                         19     7                                                    kg/10 m.sup.2     9.3    3.4                                                  10 min, lb/100 ft.sup.2                                                                         23     37                                                   kg/10 m.sup.2     11.2   18.0                                                 PV.sup.c, cp      29     45                                                   N-sec/m.sup.2 (10.sup.3)                                                                        29     45                                                   YP.sup.d, lb/100 ft.sup.2                                                                       45     16                                                   kg/10 m.sup.2     22.0   7.8                                                  HTHP fluid loss.sup.e, ml                                                                       2.8    2.8                                                  ES.sup.f, volts   814    593                                                  API fluid loss, ml                                                                              1.5    0                                                    Cake              0      0                                                    ______________________________________                                         .sup.a Dial readings were obtained using a 115volt motor driven viscomete     and measured according to the procedure described in API, pages 9-10,         sections 2.4 to 2.5.                                                          .sup.b Gel strength for 10 seconds and 10 minutes was determined in           accordance with the procedure discussed in API, page 10, section 2.5,         paragraphs f and g, respectively.                                             .sup.c PV was determined in accordance with the procedure and calculation     discussed in API, page 10, sections 2.5 to 2.6.                               .sup.d YP was determined in accordance with the procedure and calculation     discussed in API, page 10, sections 2.5 to 2.6.                               .sup.e. HTHP denotes "hightemperature/high-pressure test" and was             determined in accordance with the procedure discussed in API, pages 13-14     section 3.                                                                    .sup.f. "ES" denotes "electrical stability" and was measured in accordanc     with the procedure discussed in API, pages 21-22, section 6.             

Example 10 Toxicity Study

with one modification, another drilling fluid was prepared in accordancewith the protocol set forth in preceding Example 9 using the isoparaffinsynthetic fluid analyzed in Example 9 as the synthetic fluid. The solomodification consisted of using about ten times the amount of eachingredient in formulating the drilling fluid. The drilling fluid wassubjected to the 96 hour LC₅₀ Mysid shrimp (Mysidopuis bahia) bioassaytest by an independent laboratory and achieved a score of about 396×10³.cl Example 11

Comparative Toxicity Study

With two modifications, another drilling fluid was prepared inaccordance with the protocol set forth above in Example 9. Onemodification entailed using the diner of 1-decene (the base syntheticfluid of Novadril brand non-toxic drilling fluid) as the syntheticfluid, and the other modification consisted of using about ten times theamount of each ingredient in formulating the drilling fluid. Thedrilling fluid was subjected to the 96 hour LC₅ O Mysid shrimp(Mysidopsis bahia) bioassay test by the same independent laboratoryemployed in Example 10 and achieved a score of about 207.6×10³.

Since a higher numerical result obtained by the 96 hour LC₅₀ Mysidshrimp (Hysidopsis bahia) bioassay test is indicative of lower toxicityof the material test, comparative Examples 10-11 indicate that asynthetic fluid within the scope of the present invention issubstantially less toxic than the commercially used Novadril brandsynthetic fluid. The reason for this is that the number obtain by theexemplary synthetic fluid-containing drilling fluid is roughly about 1.9times greater than that obtained by the Novadril-containing drillingfluid. In fact, the results documented in comparative Examples 10-11 arequite surprising and unexpected because conventional wisdom in thedrilling fluids industry considers toxicity to increase with decreasingcarbon content and the tested synthetic fluid within the scope of thepresent invention has a significantly higher concentration ofhydrocarbons containing less than 20 carbon atoms than present in theNovadril brand synthetic fluid.

Example 12 Additional Analysis of Isoparaffin Synthetic Fluid Samples

Analytical results obtained from the analysis of two additionalisoparaffin synthetic fluid samples from Shell Malaysia by gaschromatography are reported in the following Table G.

                  TABLE G                                                         ______________________________________                                        Carbon Number Distribution Results                                                   Weight Percent Off                                                     Carbon   Total Organics  N-Alkanes Only.sup.a                                 Number   Sample A  Sample B  Sample A                                                                              Sample B                                 ______________________________________                                        ≦6                                                                              0.02      0.05      0.013   0.023                                    7        0.07      0.14      0.050   0.096                                    8        0.23      0.39      0.161   0.25                                     9        0.59      0.78      0.354   0.42                                     10       0.93      1.21      0.55    0.66                                     11       1.42      2.15      0.67    1.05                                     12       5.17      7.57      1.96    3.33                                     13       7.49      10.21     2.17    3.17                                     14       9.22      10.16     3.60    2.74                                     15       9.00      8.99      3.87    2.43                                     16       9.26      7.84      4.35    2.19                                     17       9.30      8.59      4.46    3.01                                     18       13.45     11.21     5.78    4.37                                     19       11.53     9.83      4.61    3.64                                     20       7.57      7.60      2.95    2.51                                     21       5.94      5.32      2.32    1.76                                     22       4.50      4.06      1.39    1.05                                     23       2.42      2.33      0.65    0.51                                     24       1.00      0.94      0.27    0.19                                     25       0.42      0.31      0.076   0.037                                    >25      0.49      0.30                                                       Total    100.02    99.98     40.254  33.436                                   FP.sup.b,                                                                     ° C.(° F.) 110(231) 106(224)                                    ______________________________________                                         .sup.a "(NAlkanes/To)100%" denotes, for a given carbon number, the weight     percent of off nalkanes divided by the weight percent off of total            organics, the quotient being multiplied by 100%.                              .sup.b "FP" denotes flash point as determined by Cleveland Open Cup           method.                                                                  

Although the present invention has been described in detail withreference to some preferred versions, other versions are possible. Forexample, the synthetic fluid can also be employed as the base liquidcomponent in other wellbore fluids. (As used in the specification andclaims, the term "wellbore fluid" means a fluid used while conductingpay zone drilling, underreaming, drilling in, plugging back, sandcontrol, perforating, gravel packing, chemical treatment, hydraulicfracturing, cleanout, well killing, tubing and hardware replacement, andzone selective operations (e.g., well completion operations) as well asa fluid employed as a packer fluid or as a spotting fluid.) In additionto the base liquid, the wellbore fluids contain on* or more additionalingredients such as proppants suitable for use in hydraulicallyfracturing subterranean formations, particulate agents suitable for usein forming a gravel pack, viscosifiers, organophilic clays, and fluidloss control agents.

Common proppants suitable for use in hydraulic fracturing procedures arequartz sand grains, tempered glass beads, sintered bauxite, resin coatedsand, aluminum pellets, and nylon pellets. Generally, the proppants areemployed in the wellbore fluids intended for use as hydraulic fracturingfluids and are used in concentrations of roughly about 1 to about 10pounds per gallon of the wellbore fluid. The proppant size is typicallysmaller than about 2 mesh on the U.S. Sieve Series scale, with the exactsize selected being dependent on the particular type of formation to befractured, the available pressure and pumping rates, an well as otherfactors known to those skilled in the art.

Typical particulate agents employed in the wellbore fluids used asgravel packing fluids include, but are not limited to, quartz sandgrains, glass beads, synthetic resins, resin coated sand, walnut shells,and nylon pellets. The gravel pack particulate agents are generally usedin concentrations of about 1 to about 20 pounds per gallon of thewellbore fluid. The size of the particulate agent employed depends onthe type of subterranean formation, the average size of formationparticles, and other parameters known to those skilled in the art.Generally, particulate agents of about 8 to about 70 mesh on the U.S.Sieve Series scale are used.

Illustrative viscosifiers, organophilic clays, and fluid loss controlagents optionally used in wellbore fluids and their concentrations arethe same as discussed above in connection with drilling fluids.

The wellbore fluids are prepared by combining the synthetic fluid withany additional additive (e.g., hydraulic fracturing proppants; gravelpack particulate agents, viscosifiers, fluid loss control agents, andorganophilic clays). The synthetic fluid typically comprises at leastabout 50 weight percent of the wellbore fluid, the weight percent beingbased on the weight of all ingredients present in the wellbore fluid.Accordingly, wellbore fluids containing at least about 60, 70, 80, oreven 90 weight percent synthetic fluid are not uncommon. (In fact, insome cases, the synthetic fluid constitutes the entire wellbore fluid.)In terms of the liquid fraction of the wellbore fluid, the syntheticfluid generally comprises from about 50 to 100 weight percent of theliquids employed in wellbore fluids. For example, the synthetic fluidcan comprise at least about 60, 70, 80, or 90 weight percent of theliquid portion of the wellbore fluid.

The specific techniques used when employing the wellbore fluid aredetermined by its intended use and are analogous to methodologiesemployed when using prior art wellbore fluids for correspondingcompletion or work-over operations. For example, when the wellbore fluidin employed as a gravel packing fluid, it is typically injected into theformation in accordance with the procedure discussed in U.S. Pat. No.4,552,215, this patent being incorporated herein in its entirety byreference.

When employed as a fracturing fluid, the wellbore fluid of the presentinvention is usually injected into the formation using proceduresanalogous to those disclosed in U.S. Pat. No. 4,488,975; U.S. Pat. No.4,553,601; Howard et al., Hydraulic Fracturing, Society of PetroleumEngineers of the American Institute of Mining, Metallurgical, andPetroleum Engineers, Inc., New York, N.Y. (1970); and Allen et al.,Production Operations, Well completions, Workover, and Stimulation, 3rdEdition, Oil & Gas Consultants International, Inc., Tulsa, Okla. (1989)(Allen), volume 2, chapter 8; these patents and publications beingincorporated herein in their entirety by reference.

When employed in a perforating operation, the wellbore fluid of thepresent invention is used, according to the methodologies disclosed involume 1, chapter 7 of Allen, referenced above.

Techniques for using packer fluids and well killing fluids, such asthose discussed in volum 1, chapter 8 of Allen, are also applicable tothe wellbore fluid of the present invention.

In addition, because the synthetic fluids of the present invention arelubricous, they can constitute up to about 10, and preferably from about2 to about 5, weight percent of a water-based drilling fluid. In fact,any moving parts can be lubricated with these synthetic fluids.

Furthermore, while the synthetic fluid in generally manufactured by theFischer-Tropsch process and various modifications thereof, fluidsmeeting the specifications set forth above in Tables I-V can also beobtained by further processing various petroleum refinery products(e.g., subjecting a petroleum product to further distillation,hydroisomerization, and/or hydrocracking procedures)

In view of the foregoing numerous other embodiments, the spirit andscope of the appended claims should not necessarily be limited to thedescription of the preferred versions contained herein.

What is claimed is:
 1. A system for drilling a well comprising:(A) atleast one subterranean formation; (B) a borehole penetrating a portionof at least one of the subterranean formations; (C) a drill bitsuspended in the borehole; and (D) a drilling fluid located in theborehole and proximate the drill bit, where the drilling fluid isnon-toxic and comprises:(I) at least one additive selected from thegroup consisting of emulsifiers, wetting agents, viscosifiers, weightingagents, and fluid loss control agents; and (II) a base fluid comprisinga paraffinic mixture that comprises (a) a total combined concentrationof iso-paraffins and n-paraffins of at least about 90 weight percent,(b) greater than 50 weight percent iso-paraffins, (c) 1 or less weightpercent naphthenics, (d) 0.1 or less weight percent aromatics, (e) atleast 1 weight percent hydrocarbons containing 11 or less carbon atoms,(f) at least 5 weight percent hydrocarbons containing 13 or less carbonatoms, and (g) at least two hydrocarbons containing a consecutive numberof carbon atoms.
 2. The system of claim 1 where the paraffinic mixturecomprises at least 55 weight percent iso-paraffins.
 3. The system ofclaim 1 where the paraffinic mixture comprises at least 60 weightpercent iso-paraffins.
 4. The system of claim 1 where the drilling fluidyields a 96 hour LC₅₀ Mysid shrimp bioassay test result greater than100,000.
 5. The system of claim 1 where the base fluid comprises about40 to about 100 volume percent paraffinic mixture and up to about 60volume percent water and the drilling fluid yields a 96 hour LC₅₀ Mysidshrimp bioassay test result greater than 100,000.
 6. The system of claim1 where the paraffinic mixture comprises at least 10 weight percenthydrocarbons containing 13 or less carbon atoms.
 7. The system of claim1 where the base fluid comprises about 40 to about 100 volume percentparaffinic mixture and up to about 60 volume percent water and theparaffinic mixture comprises at least 10 weight percent hydrocarbonscontaining 13 or less carbon atoms.
 8. The system of claim 1 where theparaffinic mixture comprises at least 15 weight percent hydrocarbonscontaining 13 or less carbon atoms.
 9. The system of claim 1 where thebase fluid comprises about 40 to about 100 volume percent paraffinicmixture and up to about 60 volume percent water and the paraffinicmixture comprises at least 15 weight percent hydrocarbons containing 13or less carbon atoms.
 10. The system of claim 1 where the paraffinicmixture comprises at least 20 weight percent hydrocarbons containing 13or less carbon atoms.
 11. The system of claim 1 where the base fluidcomprises about 40 to about 100 volume percent paraffinic mixture and upto about 60 volume percent water and the paraffinic mixture comprises atleast 20 weight percent hydrocarbons containing 13 or less carbon atoms.12. The system of claim 1 where the paraffinic mixture comprises atleast 2 weight percent hydrocarbons containing 11 or less carbon atoms.13. The system of claim 1 where the base fluid comprises about 40 toabout 100 volume percent paraffinic mixture and up to about 60 volumepercent water and the paraffinic mixture comprises at least 2 weightpercent hydrocarbons containing 11 or less carbon atoms.
 14. The systemof claim 1 where the paraffinic mixture comprises at least 3 weightpercent hydrocarbons containing 11 or less carbon atoms.
 15. The systemof claim 1 where the base fluid comprises about 40 to about 100 volumepercent paraffinic mixture and up to about 60 volume percent water andthe paraffinic mixture comprises at least 3 weight percent hydrocarbonscontaining 11 or less carbon atoms.
 16. The system of claim 1 where theparaffinic mixture comprises at least 4 weight percent hydrocarbonscontaining 11 or less carbon atoms.
 17. The system of claim 1 where thebase fluid comprises about 40 to about 100 volume percent paraffinicmixture and up to about 60 volume percent water and the paraffinicmixture comprises at least 4 weight percent hydrocarbons containing 11or less carbon atoms.
 18. The system of claim 1 where the paraffinicmixture comprises at least 5 weight percent n-paraffins.
 19. The systemof claim 1 where the paraffinic mixture comprises at least 14hydrocarbons having a consecutive number of carbon atoms.
 20. The systemof claim 1 where the paraffinic mixture comprises at least 18hydrocarbons having a consecutive number of carbon atoms.
 21. The systemof claim 1 where the paraffinic mixture comprises at least 19hydrocarbons having a consecutive number of carbon atoms.
 22. The systemof claim 1 where the paraffinic mixture comprises at least 4 groups ofparaffins,where each group of paraffins contains two or more paraffinshaving the same number of carbon atoms, the groups form a series havinga consecutive number of carbon atoms, and each of the groups contains atleast one n-paraffin and at least one iso-paraffin.
 23. The system ofclaim 1 where the paraffinic mixture comprises at least 14 groups ofparaffins,where each group of paraffins contains two or more paraffinshaving the same number of carbon atoms, the groups form a series havinga consecutive number of carbon atoms, and each of the groups contains atleast one n-paraffin and at least one iso-paraffin.
 24. The system ofclaim 1 where the paraffinic mixture comprises at least 18 groups ofparaffins,where each group of paraffins contains two or more paraffinshaving the same number of carbon atoms, the groups form a series havinga consecutive number of carbon atoms, and each of the groups contains atleast one n-paraffin and at least one iso-paraffin.
 25. The system ofclaim 1 where the paraffinic mixture comprises at least 19 groups ofparaffins,where each group of paraffins contains two or more paraffinshaving the same number of carbon atoms, the groups form a series havinga consecutive number of carbon atoms, and each of the groups contains atleast one n-paraffin and at least one iso-paraffin.
 26. The system ofclaim 1 where the paraffinic mixture is manufactured synthetically. 27.The system of claim 1 where the paraffinic mixture is a Fischer-Tropschsynthesis reaction product.
 28. The system of claim 1 where the basefluid comprises about 40 to about 100 volume percent hydrocarbons and upto about 60 volume percent water and the hydrocarbons consistessentially of the paraffinic mixture.
 29. The system of claim 1 wherethe paraffinic mixture comprises at least 6 groups of paraffins P₁, P₂,. . . P_(n-x), . . . P_(n),where each group of paraffins contains one ormore paraffins having the same number of carbon atoms, the groups form aseries having a consecutive number of carbon atoms, P₁ is the firstgroup of paraffins in the consecutive series, P₂ is the second group ofparaffins in the consecutive series, P_(n) is the last group ofparaffins in the consecutive series, x is an integer less than n,P_(n-x) has a hydrocarbon weight percent greater than any other group ofparaffins in the consecutive range, at least 72% of the groups ofparaffins in the portion of the consecutive range from P₁ to P_(n-x)have a hydrocarbon weight percent greater than that of its predecessorgroup of paraffins, and at least 72% of the groups of paraffins in theportion of the consecutive range from P_(n-x) to P_(n) have ahydrocarbon weight percent less than that of its predecessor group ofparaffins.
 30. The system of claim 29 whereat least 90% of the groups ofparaffins in the portion of the consecutive range from P₁ to P_(n-x)have a hydrocarbon weight percent greater than that of its predecessorgroup of paraffins, and at least 90% of the groups of paraffins in theportion of the consecutive range from P_(n-x) to P_(n) have ahydrocarbon weight percent less than that of its predecessor group ofparaffins.
 31. The system of claim 29 where the paraffinic mixturecomprises at least 14 groups of paraffins.
 32. The system of claim 29where the paraffinic mixture comprises at least 18 groups of paraffins.33. A system for drilling a well comprising:(A) at least onesubterranean formation; (B) a borehole penetrating a portion of at leastone of the subterranean formations; (C) a drill bit suspended in theborehole; and (D) a drilling fluid located in the borehole and proximatethe drill bit, where the drilling fluid comprises:(I) at least oneadditive selected from the group consisting of emulsifiers, wettingagents, viscosifiers, weighting agents, and fluid loss control agents;and (II) a base fluid comprising a paraffinic mixture that comprises (a)a total combined concentration of iso-paraffins and n-paraffins of atleast about 90 weight percent, (b) at least 50 weight percentiso-paraffins, (c) at least 5 weight percent hydrocarbons containing 13or less carbon atoms, and (d) at least two hydrocarbons containing aconsecutive number of carbon atoms.
 34. A system for drilling a wellcomprising:(A) at least one subterranean formation; (B) a boreholepenetrating a portion of at least one of the subterranean formations;(C) a drill bit suspended in the borehole; and (D) a drilling fluidlocated in the borehole and proximate the drill bit, where the drillingfluid comprises:(I) at least one additive selected from the groupconsisting of emulsifiers, wetting agents, viscosifiers, weightingagents, and fluid loss control agents; and (II) a base fluid comprisinga paraffinic mixture that comprises (a) a total combined concentrationof iso-paraffins and n-paraffins of at least about 90 weight percent,(b) less than 0.1 weight percent aromatics, and (c) at least 18hydrocarbons containing a consecutive number of carbon atoms.
 35. Asystem for drilling a well comprising:(A) at least one subterraneanformation; (B) a borehole penetrating a portion of at least one of thesubterranean formations; (C) a drill bit suspended in the borehole; and(D) a drilling fluid located in the borehole and proximate the drillbit, where the drilling fluid comprises:(I) at least one additiveselected from the group consisting of emulsifiers, wetting agents,viscosifiers, weighting agents, and fluid loss control agents; and (II)a base fluid comprising a paraffinic mixture that comprises (a) a totalcombined concentration of iso-paraffins and n-paraffins of at leastabout 90 weight percent, (b) less than 0.1 weight percent aromatics, (c)at least 5 weight percent hydrocarbons containing 13 or less carbonatoms, and (d) at least 14 hydrocarbons containing a consecutive numberof carbon atoms.
 36. A system for drilling a well comprising:(A) atleast one subterranean formation; (B) a borehole penetrating a portionof at least one of the subterranean formations; (C) a drill bitsuspended in the borehole; and (D) a drilling fluid located in theborehole and proximate the drill bit, where the drilling fluidcomprises:(I) at least one additive selected from the group consistingof emulsifiers, wetting agents, viscosifiers, weighting agents, andfluid loss control agents; and (II) a base fluid comprising a pluralityof hydrocarbons, where the hydrocarbons comprise (a) a total combinedconcentration of iso-paraffins and n-paraffins of at least 96 weightpercent, (b) at least 1 weight percent hydrocarbons containing 11 orless carbon atoms, (c) at least 5 weight percent hydrocarbons containing13 or less carbon atoms, (d) at least two hydrocarbons containing aconsecutive number of carbon atoms, and (e) at least 2 weight percentiso-paraffins.
 37. The system of claim 36 where the hydrocarbonscomprise a total combined concentration of iso-paraffins and n-paraffinsof at least 97 weight percent.
 38. The system of claim 36 where thehydrocarbons comprise a total combined concentration of iso-paraffinsand n-paraffins of at least 98 weight percent.
 39. The system of claim36 where the hydrocarbons comprise a total combined concentration ofiso-paraffins and n-paraffins of at least 99 weight percent.
 40. Asystem for drilling a well comprising:(A) at least one subterraneanformation; (B) a borehole penetrating a portion of at least one of thesubterranean formations; (C) a drill bit suspended in the borehole; and(D) a drilling fluid located in the borehole and proximate the drillbit, where the drilling fluid comprises:(I) at least one additiveselected from the group consisting of emulsifiers, wetting agents,viscosifiers, weighting agents, and fluid loss control agents; and (II)a base fluid comprising a plurality of hydrocarbons, where thehydrocarbons comprise (a) a total combined concentration ofiso-paraffins and n-paraffins of greater than 90 weight percent, (b) atleast 1 weight percent hydrocarbons containing 11 or less carbon atoms,(c) at least 5 weight percent hydrocarbons containing 13 or less carbonatoms, (d) at least ten hydrocarbons containing a consecutive number ofcarbon atoms, and (e) at least 2 weight percent iso-paraffins.
 41. Thesystem of claim 36 where the hydrocarbons comprise at least 14hydrocarbons containing a consecutive number of carbon atoms.
 42. Thesystem of claim 36 where the hydrocarbons comprise at least 18hydrocarbons containing a consecutive number of carbon atoms.
 43. Asystem for drilling a well comprising:(A) at least one subterraneanformation; (B) a borehole penetrating a portion of at least one of thesubterranean formations; (C) a drill bit suspended in the borehole; and(D) a drilling fluid located in the borehole and proximate the drillbit, where the drilling fluid comprises:(I) at least one additiveselected from the group consisting of emulsifiers, wetting agents,viscosifiers, weighting agents, and fluid loss control agents; and (II)a base fluid comprising a paraffinic mixture that comprises (a) at leastabout 90 weight percent paraffins selected from the group consisting ofn-paraffins, iso-paraffins, and mixtures thereof, (b) 0.1 or less weightpercent aromatics, and (c) at least 4 groups of paraffins, whereeachgroup of paraffins contains two or more paraffins having the same numberof carbon atoms, the groups form a series having a consecutive number ofcarbon atoms, and each of the groups contains at least one n-paraffinand at least one iso-paraffin.
 44. The system of claim 43 where theparaffinic mixture comprises at least 10 groups of paraffins,where eachgroup of paraffins contains two or more paraffins having the same numberof carbon atoms, the groups form a series having a consecutive number ofcarbon atoms, and each of the groups contains at least one n-paraffinand at least one iso-paraffin.
 45. A system for drilling a wellcomprising:(A) at least one subterranean formation; (B) a boreholepenetrating a portion of at least one of the subterranean formations;(C) a drill bit suspended in the borehole; and (D) a drilling fluidlocated in the borehole and proximate the drill bit, where the drillingfluid comprises:(I) at least one additive selected from the groupconsisting of emulsifiers, wetting agents, viscosifiers, weightingagents, and fluid loss control agents; and (II) a base fluid comprisinga paraffinic mixture that comprises (a) at least about 90 weight percentparaffins selected from the group consisting of n-paraffins,iso-paraffins, and mixtures thereof, (b) 0.1 or less weight percentaromatics, and (c) at least 8 groups of paraffins having a consecutivenumber of carbon atoms, whereeach group of paraffins contains one ormore paraffins having the same number of carbon atoms; then-paraffin/iso-paraffin weight ratio is greater than 1 for at least halfof the groups of paraffins within at least the first quarter of theconsecutive range of paraffins; the n-paraffin/iso-paraffin weight ratiois less than 1 for at least half of the groups of paraffins within atleast the last quarter of the consecutive range of paraffins; and thefirst quarter or other first fraction contains the groups of paraffinscontaining the lower number of carbon atoms and the last quarter orother last fraction contains the groups of paraffins containing thehigher number of carbon atoms.
 46. The system of claim 45 where theparaffinic mixture comprises 8 groups of paraffins having a consecutivenumber of carbon atoms,where the n-paraffin/iso-paraffin weight ratio isgreater than 1 for at least one of the two groups of paraffins withinthe first quarter of the consecutive range of paraffins; and then-paraffin/iso-paraffin weight ratio is less than 1 for at least one ofthe two groups of paraffins within the last quarter of the consecutiverange of paraffins.
 47. The system of claim 45 where the paraffinicmixture comprises 14 groups of paraffins having a consecutive number ofcarbon atoms,where the n-paraffin/iso-paraffin weight ratio is greaterthan 1 for at least two of the four groups of paraffins within the firstquarter of the consecutive range of paraffins; and then-paraffin/iso-paraffin weight ratio is less than 1 for at least two ofthe four groups of paraffins within the last quarter of the consecutiverange of paraffins.
 48. The system of claim 45 where the paraffinicmixture comprises 14 groups of paraffins having a consecutive number ofcarbon atoms,where the n-paraffin/iso-paraffin weight ratio is greaterthan 1 for at least three of the four groups of paraffins within thefirst quarter of the consecutive range of paraffins; and then-paraffin/iso-paraffin weight ratio is less than 1 for at least threeof the four groups of paraffins within the last quarter of theconsecutive range of paraffins.
 49. The system of claim 45 where theparaffinic mixture comprises 18 groups of paraffins having a consecutivenumber of carbon atoms,where the n-paraffin/iso-paraffin weight ratio isgreater than 1 for at least three of the five groups of paraffins withinthe first quarter of the consecutive range of paraffins; and then-paraffin/iso-paraffin weight ratio is less than 1 for at least threeof the five groups of paraffins within the last quarter of theconsecutive range of paraffins.
 50. The system of claim 45 where theparaffinic mixture comprises 18 groups of paraffins having a consecutivenumber of carbon atoms,where the n-paraffin/iso-paraffin weight ratio isgreater than 1 for at least four of the five groups of paraffins withinthe first quarter of the consecutive range of paraffins; and then-paraffin/iso-paraffin weight ratio is less than 1 for at least four ofthe five groups of paraffins within the last quarter of the consecutiverange of paraffins.
 51. The system of claim 45 where the paraffinicmixture comprises 18 groups of paraffins having a consecutive number ofcarbon atoms,where the n-paraffin/iso-paraffin weight ratio is greaterthan 1 for at least three of the six groups of paraffins within thefirst third of the consecutive range of paraffins; and then-paraffin/iso-paraffin weight ratio is less than 1 for at least threeof the six groups of paraffins within the last third of the consecutiverange of paraffins.
 52. The system of claim 45 where the paraffinicmixture comprises 18 groups of paraffins having a consecutive number ofcarbon atoms,where the n-paraffin/iso-paraffin weight ratio is greaterthan 1 for at least four of the six groups of paraffins within the firstthird of the consecutive range of paraffins; and then-paraffin/iso-paraffin weight ratio is less than 1 for at least four ofthe six groups of paraffins within the last third of the consecutiverange of paraffins.
 53. The system of claim 45 where the paraffinicmixture comprises 19 groups of paraffins having a consecutive number ofcarbon atoms,where the n-paraffin/iso-paraffin weight ratio is greaterthan 1 for at least three of the five groups of paraffins within thefirst quarter of the consecutive range of paraffins; and then-paraffin/iso-paraffin weight ratio is less than 1 for at least threeof the five groups of paraffins within the last quarter of theconsecutive range of paraffins.
 54. The system of claim 45 where theparaffinic mixture comprises 19 groups of paraffins having a consecutivenumber of carbon atoms,where the n-paraffin/iso-paraffin weight ratio isgreater than 1 for at least four of the five groups of paraffins withinthe first quarter of the consecutive range of paraffins; and then-paraffin/iso-paraffin weight ratio is less than 1 for at least four ofthe five groups of paraffins within the last quarter of the consecutiverange of paraffins.
 55. The system of claim 45 where each of theconsecutive groups comprises at least one n-paraffin and at least oneiso-paraffin.
 56. A natural resource system comprising:(a) asubterranean formation; (b) a well penetrating at least a portion of thesubterranean formation; and (c) a drilling fluid present in at least aportion of the well, where the drilling fluid is non-toxic andcomprises:(I) at least one additive selected from the group consistingof emulsifiers, wetting agents, viscosifiers, weighting agents, andfluid loss control agents; and (II) a base fluid comprising a paraffinicmixture that comprises (a) a total combined concentration ofiso-paraffins and n-paraffins of at least about 90 weight percent, (b)greater than 50 weight percent iso-paraffins, (c) 1 or less weightpercent naphthenics, (d) 0.1 or less weight percent aromatics, (e) atleast 1 weight percent hydrocarbons containing 11 or less carbon atoms,(f) at least 5 weight percent hydrocarbons containing 13 or less carbonatoms, and (g) at least two hydrocarbons containing a consecutive numberof carbon atoms.
 57. A system for drilling a well comprising:(A ) atleast one subterranean formation; (B) a borehole penetrating a portionof at least one of the subterranean formations; (C) a drill bitsuspended in the borehole; and (D) a drilling fluid located in theborehole and proximate the drill bit, where the drilling fluidcomprises:(I) at least one additive selected from the group consistingof emulsifiers, wetting agents viscosifiers, weighting agents, and fluidloss control agents; and (II) a base fluid comprising a paraffinicmixture that comprises (a) a total concentration of iso-paraffins andn-paraffins greater than 90 weight percent (b) at least 1 weight percenthydrocarbons containing 11 or less carbon atoms, (c) at least 5 weightpercent hydrocarbons containing 13 or less carbon atoms, (d) at least 10hydrocarbons containing a consecutive number of carbon atoms, and (e)greater than 2 weight percent iso-paraffins.
 58. The system of claim 57where the paraffinic mixture comprises at least 14 hydrocarbonscontaining a consecutive number of carbon atoms.
 59. The system of claim57 where the paraffinic mixture comprises at least 18 hydrocarbonscontaining a consecutive number of carbon atoms.
 60. A system fordrilling a well comprising:(A) at least one subterranean formation; (B)a borehole penetrating a portion of at least one of the subterraneanformations; (C) a drill bit suspended in the borehole; and (D) adrilling fluid located in the borehole and proximate the drill bit,where the drilling fluid is non-toxic and comprises:(I) at least oneadditive selected from the group consisting of emulsifiers, wettingagents, viscosifiers, weighting agents, and fluid loss control agents;and (II) a base fluid comprising a paraffinic mixture that comprises (a)a total combined concentration of iso-paraffins and n-paraffins of atleast 90 weight percent, (b) at least 1 weight percent hydrocarbonscontaining 11 or less carbon atoms, (c) at least 5 weight percenthydrocarbons containing 13 or less carbon atoms, and (d) at least tenhydrocarbons containing a consecutive number of carbon atoms.
 61. Thesystem of claim 60 where the paraffinic mixture comprises at least 14hydrocarbons containing a consecutive number of carbon atoms.
 62. Thesystem of claim 60 where the paraffinic mixture comprises at least 18hydrocarbons containing a consecutive number of carbon atoms.
 63. Asystem for drilling a well comprising:(A) at least one subterraneanformation; (B) a borehole penetrating a portion of at least one of thesubterranean formations; (C) a drill bit suspended in the borehole; and(D) a drilling fluid located in the borehole and proximate the drillbit, where the drilling fluid is non-toxic and comprises:(I) at leastone additive selected from the group consisting of emulsifiers, wettingagents, viscosifiers, weighting agents, and fluid loss control agents;and (II) a base fluid comprising a plurality of hydrocarbons, where thehydrocarbons comprise (a) a total combined concentration ofiso-paraffins and n-paraffins of at least 95 weight percent, (b) atleast 1 weight percent hydrocarbons containing 11 or less carbon atoms,(c) at least 5 weight percent hydrocarbons containing 13 or less carbonatoms, and (d) at least two hydrocarbons containing a consecutive numberof carbon atoms.
 64. The system of claim 63 where the hydrocarbonscomprise a total combined concentration of iso-paraffins and n-paraffinsof at least 96 weight percent.
 65. The system of claim 63 where thehydrocarbons comprise a total combined concentration of iso-paraffinsand n-paraffins of at least 97 weight percent.
 66. The system of claim63 where the hydrocarbons comprise a total combined concentration ofiso-paraffins and n-paraffins of at least 98 weight percent.
 67. Thesystem of claim 63 where the hydrocarbons comprise a total combinedconcentration of iso-paraffins and n-paraffins of at least 99 weightpercent.
 68. A system for drilling a well comprising:(A) at least onesubterranean formation; (B) a borehole penetrating a portion of at leastone of the subterranean formations; (C) a drill bit suspended in theborehole; and (D) a drilling fluid located in the borehole and proximatethe drill bit, where the drilling fluid comprises:(I) at least oneadditive selected from the group consisting of emulsifiers, wettingagents, viscosifiers, weighting agents, and fluid loss control agents;and (II) a non-toxic base fluid comprising a mixture of hydrocarbonshaving a pour point greater than about -30° C., said mixture comprising(a) at least 95 weight percent of hydrocarbons containing 11 or morecarbon atoms; (b) greater than 5 weight percent of hydrocarbonscontaining 18 or more carbon atoms; (c) at least 50 weight percentiso-paraffins; (d) at least 90 weight percent total paraffins; (e) atleast 10 hydrocarbons containing a consecutive number of carbon atoms;(f) less than about 1 weight percent naphthenics; (g) less than about0.1 volume percent aromatics; and (h) at least some hydrocarbonscontaining 13 carbon atoms.
 69. A system for drilling a wellcomprising:(A) at least one subterranean formation; (B) a boreholepenetrating a portion of at least one of the subterranean formations;(C) a drill bit suspended in the borehole; and (D) a drilling fluidlocated in the borehole and proximate the drill bit, where the drillingfluid comprises:(I) at least one additive selected from the groupconsisting of emulsifiers, wetting agents, viscosifiers, weightingagents, and fluid loss control agents; and (II) a non-toxic base fluidcomprising a mixture of hydrocarbons having a pour point greater thanabout -30° C., said mixture comprising (a) at least 95 weight percent ofhydrocarbons containing 11 or more carbon atoms; (b) greater than 5weight percent of hydrocarbons containing 18 or more carbon atoms; (c)at least 50 weight percent iso-paraffins; (d) at least 96 weight percenttotal paraffins; (e) at least 2 hydrocarbons containing a consecutivenumber of carbon atoms; (f) less than about 1 weight percentnaphthenics; (g) less than about 0.1 volume percent aromatics; and (h)at least some hydrocarbons containing 13 carbon atoms.
 70. A system asdefined in claim 69 wherein said mixture of hydrocarbons issubstantially completely synthetic.
 71. A system as defined in claim 68wherein said mixture of hydrocarbons further comprises at least 55weight percent iso-paraffins.
 72. A system as defined in claim 71wherein said mixture of hydrocarbons comprises at least 10 weightpercent n-paraffins.
 73. A system as defined in claim 71 wherein saidmixture of hydrocarbons comprises at least some hydrocarbons containing11 carbon atoms.
 74. A system as defined in claim 71 wherein saidmixture of hydrocarbons comprises at least fourteen hydrocarbonscontaining a consecutive number of carbon atoms.
 75. A system fordrilling a well comprising:(A) at least one subterranean formation; (B)a borehole penetrating a portion of at least one of the subterraneanformations; (C) a drill bit suspended in the borehole; and (D) adrilling fluid located in the borehole and proximate the drill bit,where the drilling fluid comprises:(I) at least one additive selectedfrom the group consisting of emulsifiers, wetting agents, viscosifiers,weighting agents, and fluid loss control agents; and (II) a non-toxic,substantially completely synthetic base fluid comprising a mixture ofhydrocarbons having a pour point greater than about -30° C., saidmixture comprising (a) at least 20 weight percent n-paraffins; (b) atleast 95 weight percent of hydrocarbons containing 1l or more carbonatoms; (c) greater than 5 weight percent of hydrocarbons containing 18or more carbon atoms; (d) at least 55 weight percent iso-paraffins; (e)at least 90 weight percent total paraffins; (f) at least sixhydrocarbons containing a consecutive number of carbon atoms; (q) lessthan about 1 weight percent naphthenics; (h) less than about 0.1 volumepercent aromatics; and (i) at least some hydrocarbons containing 13carbon atoms.
 76. A system as defined in claim 75 wherein said mixtureof hydrocarbons comprises at least some hydrocarbons containing 11carbon atoms.
 77. A system as defined in claim 75 wherein said mixtureof hydrocarbons comprises at least fourteen hydrocarbons containing aconsecutive number of carbon atoms.
 78. A system as defined in claim 69wherein said mixture of hydrocarbons has an end boiling point no greaterthan 375° C. and comprises at least 55 weight percent iso-paraffins. 79.A system as defined in claim 78 wherein said mixture of hydrocarbonscomprises at least 20 weight percent n-paraffins and at least eighthydrocarbons containing a consecutive number of carbon atoms.
 80. Asystem as defined in claim 79 wherein said mixture of hydrocarbonscomprises at least ten hydrocarbons containing a consecutive number ofcarbon atoms.
 81. A system as defined in claim 80 wherein said mixtureof hydrocarbons comprises at least some hydrocarbons containing 11carbon atoms.
 82. A system as defined in claim 80 wherein said mixtureof hydrocarbons comprises at least 25 weight percent n-paraffins.
 83. Asystem as defined in claim 68 wherein said mixture of hydrocarbonscomprises at least fourteen hydrocarbons containing a consecutive numberof carbon atoms.
 84. A system as defined in claim 83 wherein saidmixture of hydrocarbons has an end boiling point no greater than 375° C.and comprises at least 30 weight percent n-paraffins and at least 55weight percent iso-paraffins.
 85. A system as defined in claim 84wherein said mixture of hydrocarbons comprises at least somehydrocarbons containing 11 carbon atoms.
 86. A system for drilling awell comprising:(A) at least one subterranean formation; (B) a boreholepenetrating a portion of at least one of the subterranean formations;(C) a drill bit suspended in the borehole; and (D) a drilling fluidlocated in the borehole and proximate the drill bit, where the drillingfluid comprises:(I) at least one additive selected from the groupconsisting of emulsifiers, wetting agents, viscosifiers, weightingagents, and fluid loss control agents; and (II) a non-toxic base fluidcomprising a mixture of hydrocarbons having a pour point greater thanabout -30° C., said mixture comprising (a) at least 95 weight percent ofhydrocarbons containing 11 or more carbon atoms; (b) greater than 5weight percent of hydrocarbons containing 18 or more carbon atoms; (c)at least 50 weight percent iso-paraffins; (d) at least 90 weight percenttotal paraffins; (e) at least 18 hydrocarbons containing a consecutivenumber of carbon atoms; (f) less than about 1 weight percentnaphthenics; and (g) less than about 0.1 volume percent aromatics.
 87. Asystem as defined in claim 86 wherein said mixture of hydrocarbons issubstantially completely synthetic.
 88. A system as defined in claim 86wherein said mixture of hydrocarbons comprises at least 96 weightpercent total paraffins.
 89. A system as defined in claim 86 whereinsaid mixture of hydrocarbons comprises at least 96 weight percent totalparaffins.
 90. A system as defined in claim 88 wherein said mixture ofhydrocarbons comprises at least some hydrocarbons containing 11 carbonatoms.
 91. A system as defined in claim 89 wherein said mixture ofhydrocarbons comprises at least 55 weight percent iso-paraffins and atleast 10 weight percent n-paraffins.
 92. A system as defined in claim 91wherein said mixture of hydrocarbons comprises at least somehydrocarbons containing 11 carbon atoms.
 93. A system as defined inclaim 91 wherein said mixture of hydrocarbons comprises at least 20weight percent n-paraffins.
 94. A system as defined in claim 88 whereinsaid mixture of hydrocarbons comprises at least 15 weight percentn-paraffins and at least 55 weight percent iso-paraffins and has an endboiling point no greater than 375° C.
 95. A system as defined in claim94 wherein said mixture of hydrocarbons comprises at least somehydrocarbons containing 11 carbon atoms and further comprises at least25 weight percent n-paraffins.
 96. A system as defined in claim 95wherein said mixture of hydrocarbons comprises at least 35 weightpercent n-paraffins.
 97. A system as defined in claim 87 wherein saidmixture of hydrocarbons comprises at least some hydrocarbons containing13 carbon atoms.
 98. A system as defined in claim 97 wherein saidmixture of hydrocarbons comprises at least 55 weight percentiso-paraffins and at least 10 weight percent n-paraffins.
 99. A systemas defined in claim 98 wherein said mixture of hydrocarbons comprises atleast some hydrocarbons containing 11 carbon atoms.
 100. A system asdefined in claim 98 wherein said mixture of hydrocarbons comprises atleast 25 weight percent n-paraffins and has an end boiling point nogreater than 375° C.
 101. A system as defined in claim 100 wherein saidmixture of hydrocarbons comprises at least some hydrocarbons containing11 carbon atoms.
 102. A system as defined in claim 101 wherein saidmixture of hydrocarbons comprises at least 35 weight percentn-paraffins.
 103. A system for drilling a well comprising:(A) at leastone subterranean formation; (B) a borehole penetrating a portion of atleast one of the subterranean formations; (C) a drill bit suspended inthe borehole; and (D) a drilling fluid located in the borehole andproximate the drill bit, where the drilling fluid comprises:(I) a atleast one additive selected from the group consisting of emulsifiers,wetting agents, viscosifiers, weighting agents, and fluid loss controlagents; and (II) a non-toxic base fluid comprising a mixture ofhydrocarbons, said hydrocarbons consisting essentially of paraffins andcomprising (a) at least 95 weight percent of hydrocarbons containing 10or more carbon atoms; (b) at least 90 weight percent n-paraffins; (c) atleast some hydrocarbons containing 13 carbon atoms; and (d) at least 2hydrocarbons containing a consecutive number of carbon atoms.
 104. Asystem as defined in claim 103 wherein said mixture of hydrocarbons issubstantially completely synthetic.
 105. A system for drilling a wellcomprising:(A) at least one subterranean formation; (B) a boreholepenetrating a portion of at least one of the subterranean formations;(C) a drill bit suspended in the borehole; and (D) a drilling fluidlocated in the borehole and proximate the drill bit, where the drillingfluid comprises:(I) at least one additive selected from the groupconsisting of emulsifiers, wetting agents, viscosifiers, weightingagents, and fluid loss control agents; and (II) a non-toxic base fluidcomprising a mixture of hydrocarbons comprising (a) at least 95 weightpercent of hydrocarbons containing 10 or more carbon atoms; (b) at least90 weight percent n-paraffins; (c) at least 2 hydrocarbons containing aconsecutive number of carbon atoms; and (d) essentially no hydrocarbonscontaining more than 18 carbon atoms.
 106. A system as defined in claim105 wherein said mixture of hydrocarbons is substantially completelysynthetic.
 107. A system as defined in claim 104 wherein said mixture ofhydrocarbons contains no more than 5 weight percent naphthenes.
 108. Asystem as defined in claim 107 wherein said mixture of hydrocarbonscontains a hydrocarbon containing 11 carbon atoms.
 109. A system asdefined in claim 105 wherein essentially no hydrocarbon in said mixtureof hydrocarbons contains less than 10 carbon atoms or more than
 18. 110.A system as defined in claim 109 wherein said mixture of hydrocarbonscontains less than 5 weight percent naphthenes, greater than 90 weightpercent n-paraffins, and at least 4 hydrocarbons containing aconsecutive number of carbon atoms.
 111. A system as defined in claim110 wherein the number of carbon atoms in essentially all of thehydrocarbons in said mixture of hydrocarbons lie in a 5 unit consecutiverange.
 112. A system as defined in claim 110 wherein the number ofcarbon atoms in essentially all of the hydrocarbons in said mixture ofhydrocarbons lie in a 6 unit consecutive range.
 113. A system as definedin claim 111 wherein said mixture of hydrocarbons contains at least 3weight percent of iso-paraffins.
 114. A system as defined in claim 110wherein said mixture of hydrocarbons contains less than 5 weight percentof iso-paraffins.
 115. A system as defined in claim 114 wherein saidmixture of hydrocarbons contains a hydrocarbon containing 11 carbonatoms.
 116. A system as defined in claim 110 wherein said mixture ofhydrocarbons contains a hydrocarbon containing 11 carbon atoms.
 117. Asystem as defined in claim 109 wherein said mixture of hydrocarbonscontains less than 5 weight percent naphthenes, at least 91 weightpercent n-paraffins, and at least 5 hydrocarbons containing aconsecutive number of carbon atoms.
 118. A system as defined in claim117 wherein said mixture of hydrocarbons contains at least 3 weightpercent of iso-paraffins.
 119. A system as defined in claim 117 whereinsaid mixture of hydrocarbons contains less than 5 weight percent ofiso-paraffins.
 120. A system as defined in claim 119 wherein saidmixture of hydrocarbons contains a hydrocarbon containing 11 carbonatoms.
 121. A system as defined in claim 117 wherein said mixture ofhydrocarbons contains a hydrocarbon containing 11 carbon atoms.
 122. Asystem as defined in claim 109 wherein said mixture of hydrocarbonscontains less than 5 weight percent naphthenes, at least 91 weightpercent n-paraffins, and at least 6 hydrocarbons containing aconsecutive number of carbon atoms.
 123. A system as defined in claim122 wherein said mixture of hydrocarbons contains at least 3 weightpercent of iso-paraffins.
 124. A system as defined in claim 122 whereinsaid mixture of hydrocarbons contains less than 5 weight percent ofiso-paraffins.
 125. A system as defined in claim 124 wherein saidmixture of hydrocarbons contains less than 0.1 volume percent aromaticsand no more than 1 weight percent naphthenes.
 126. A system as definedin claim 120 wherein said mixture of hydrocarbons contains less than 0.1volume percent aromatics and no more than 1 weight percent naphthenes.127. A system as defined in claim 109 wherein said mixture ofhydrocarbons contains less than 0.1 volume percent aromatics and no morethan 1 weight percent naphthenes.